Among calcium aluminosilicates containing CaO, Al2O3, and SiO2 as components, there are substances whose mineral name is called mayenite. Compounds having the same type of crystal structure as mayenite are referred to as “mayenite compounds”. Mayenite compounds have a typical composition represented by 12CaO.7Al2O2 (hereinafter represented by “C12A7”). It has been reported that a C12A7 crystal has a specific crystal structure in which two oxygen ions out of 66 oxygen ions in a unit cell including two molecules are clathrated in the form of “free oxygen” in a space of a cage formed by the crystal skeleton (NPL 1).
Since 2003, the inventors of the present invention disclosed that free oxygen ions included in mayenite compounds can be substituted with various types of anions. In particular, by maintaining C12A7 in a strong reducing atmosphere, all free oxygen ions can be substituted with electrons. C12A7 whose free oxygen ions are substituted with electrons can be represented by a chemical formula of [Ca24Al28O64]4+(e−)4 (hereinafter represented by “C12A7:e−”). Substances in which anions are substituted with electrons in this manner are referred to as “electrides”, and electrides have a feature of exhibiting good electron conductivity (NPL 2).
The inventors of the present invention found that C12A7:e− including conduction electrons at a concentration of 1×1019 cm−3 or more and a compound having the same type of crystal structure as C12A7 are obtained by (A) a method that includes maintaining an isostatically pressed compact of a single crystal or a fine powder of C12A7 in an alkali metal vapor or an alkaline earth metal vapor in the range of 600° C. to 800° C., (B) a method that includes performing ion-implantation of an inert ion into a C12A7 thin film, or (C) a method that includes melting an isostatically pressed compact of a fine powder of C12A7 in a reducing atmosphere, and directly solidifying the melt (PTL 1).
The inventors of the present invention have filed patents for inventions relating to a method in which a raw material substance of a good conductive mayenite compound is melted, and the resulting melt is maintained in an atmosphere with a low oxygen partial pressure and then subjected to cooling solidification (PTL 2); and a method in which a reducing agent such as carbon, Al, or Ti is added to a powder obtained by pulverizing a sintered product prepared by maintaining a raw material powder at a high temperature to sinter the raw material powder in a solid-phase reaction, a press-molded body of the powder, or a sintered body obtained by sintering the molded body in the range of 1,200° C. to 1,350° C., and the resulting mixture is heat-treated in the range of 600° C. to 1,415° C. to provide conductivity (that is, to substitute free oxygen ions with electrons) (PTL 3 and PTL 4). Furthermore, the inventors of the present invention successfully obtained C12A7:e− that exhibits a metallic electrical conduction property by annealing a C12A7 single crystal in a metallic titanium (Ti) vapor, and have filed a patent for inventions relating to a method for producing the C12A7:e− and use of the C12A7:e− as an electron emission material (PTL 5).
Patents of inventions relating to the following methods for producing a conductive mayenite compound have been filed. Examples of the methods include a method in which a compound oxide film represented by 12Ca1-xSrxO7Al2O3 (x=0 to 1), the compound oxide film being obtained by firing a raw material of a non-aqueous solution by heating in the range of 500° C. to 1,500° C., is heated in the range of 700° C. to 1,500° C. to perform a reduction treatment (PTL 6); a method in which a mixed raw material is heated in an inert atmosphere with an oxygen partial pressure of 1,000 Pa or less or in a vacuum atmosphere in the range of 1,200° C. to 1,415° C. (PTL 7); a method in which a mixture of a raw material and a reducing agent such as metallic Al or metallic Ca is sintered in the range of 1,200° C. to 1,415° C. or melted in the range of 1,415° C. to 1,600° C. (PTL 8); and a method in which a mayenite compound powder is heated in the range of 300° C. to 1,200° C. to form a sintered body having open pores, and the sintered body is heated in a reducing atmosphere in the range of 1,200° C. to 1,450° C. (PTL 9).
Regarding C12A7:e−, which has a metallic electrical conduction property, a powder of C12A7:e− can be directly synthesized by mixing CaCO3 with Al2O3 at a ratio of 11:7, heating the mixture at 1,300° C., and heating the resulting product in a metallic Ca vapor atmosphere (NPL 3). Conductive mayenite compounds are used in electron emitters, field emission display devices, cold cathode fluorescent tubes, flat lighting devices, electron emission materials (PTL 10), electrodes for discharge lamps (PTL 11), and the like.
Furthermore, a patent for an invention relating to a mayenite compound in which some Al atoms in C12A7, which is a conductive mayenite compound, are substituted with Ga or In atoms has been filed. This mayenite compound is suitable for electrode materials that require a high-temperature heat treatment, for example, a plasma display panel (PDP) protective film material, a charge injection material in an organic electroluminescent (EL) device, and the like (PTL 12).
The inventors of the present invention have filed patents for inventions relating to a catalyst of an ammonia synthesis reaction, the catalyst including a conductive mayenite compound and a metal such as Ru or Fe supported on the conductive mayenite compound (PTL 13) and a method for reducing carbon dioxide to carbon monoxide by using a conductive mayenite compound (PTL 14). Furthermore, even C12A7 that does not have electrical conductivity has applications as a catalyst or a catalyst support. For example, it is known that a catalyst obtained by spray-drying a complex solution of a raw material and subsequently calcining the resulting product in the range of 1,300° C. to 1,400° C. for two hours or more is used as a catalyst for steam cracking reaction for producing a soft olefin (PTL 15). Recently, methods for producing a support having a large specific surface area, the methods including the steps of synthesizing a precursor by a hydrothermal method or a sol-gel method and subsequently firing the precursor, have been proposed (NPL 4 and NPL 5).
It has been reported that when C12A7 is left to stand in an atmosphere containing moisture, hydroxide ions (OH−) are clathrated in the cage and are not easily removed even at high temperatures (NPL 6).